The present invention relates as indicated to a collapsible tower boom lift, and relates more particularly to a tower boom lift in which the operator's platform can reach up to substantial heights while being maintained level with respect to the ground, while at the same time permitting the platform to reach over obstacles such as machine tools, piping, tanks, and other obstacles which are normally encountered in work environments of this type and encumber the operation of the boom lift.
In machines of this type, it is desirable that the upper boom and mountings therefor be able to swing without having any part of the turntable or boom elevating mechanism extend beyond the perimeter of the chassis. This arrangement permits the boom lift to operate within relatively narrow aisleway dimensions and prevent damage to equipment and materials positioned around the lift. Previously available boom lifts were frequently characterized by having operating parts of the boom elevating mechanism extend beyond the chassis, a condition called tailswing if the extension appears at the end of the chassis away from the operator, or underswing if the extension occurred on the end of the turntable closest to the operator. In those instances where tailswing or underswing were within desired parameters, the boom lift or aerial work platform was limited in the amount of height possible under the upper boom, even in work platforms where upper and lower boom assemblies were employed, thereby disadvantageously affecting or limiting the obtainable height of the work platform.
The following are examples of prior art boom lifts which exhibit the disadvantages referred to above. In U.S. Pat. No. 4,643,273 an access lift is comprised of extensible upper and lower boom assemblies. However, the operator cannot enter the platform until the lower boom is raised and locked, thereby eliminating a principle objective of equipment of this type, specifically, to provide a self-propelled machine which can be operated from the platform. This objective cannot be accomplished in the '273 patent due to the lack of any type of levelling system to maintain the platform attitude during operation of the lower boom. Moreover, tailswing is eliminated only after the lower boom has been raised, thereby requiring a much greater space within which to operate the equipment.
U.S. Pat. No. 4,280,589 discloses an extensible upper boom assembly operatively connected to parallel support arms in turn operatively connected to a post member secured to the turntable frame. The mounting arrangement for the support arms is such that the same problem of tailswing is encountered as referred to above. The same disadvantage exists in the self-propelled aerial lift disclosed in U.S. Pat. No. 4,160,492, which discloses a single extensible boom assembly operable from controls contained in the work platfrm.
Although U.S. Pat. No. Re. 31,400 discloses a self-propelled boom lift in which tailswing is eliminated, the single boom arrangement has obvious limitations in terms of the height attainable of the work platform.